• Earth expected to be habitable for another 1.75 billion years
• Seismologists puzzle over largest deep earthquake ever recorded
SCIENTISTS might have finally laid to rest the belief that the world may soon come to an end after a doomsday or Armageddon scenario.
A recent study by astrobiologists at the University of East Anglia (UEA), United Kingdom (UK), suggests that habitable conditions on Earth will be possible for at least another 1.75 billion years.
According to the findings published earlier this week in the journal Astrobiology, the habitable lifetime of planet Earth - based on distance from the sun and temperatures at which it is possible for the planet to have liquid water.
Doomsday may refer to end time, a time period described in the eschatological writings in Abrahamic religions and in doomsday scenarios non-Abrahamic religions.
Armageddon will be, according to the Book of Revelation, the site of a battle during the end times, variously interpreted as either a literal or symbolic location. The term is also used in a generic sense to refer to any end of the world scenario.
The method used to make the calculation can also identify planets outside the Solar System with long ‘habitable periods’, which might be the best places to look for life.
The habitable zone around a star is the area in which an orbiting planet can support liquid water, the perfect solvent for the chemical reactions at the heart of life. Too far from a star and a planet’s water turns to permanent ice and its carbon dioxide condenses; too close, and the heat turns water into vapour that escapes into space.
Habitable zones are not static. The luminosity of a typical star increases as its composition and chemical reactions evolve over billions of years, pushing the habitable zone outward. Researchers reported in March that Earth is closer to the inner edge of the Sun’s habitable zone than previously thought.
The inner edge of the Sun’s habitable zone is moving outwards at a rate of about one metre per year. The latest model predicts a total habitable zone lifetime for Earth of 6.3 billion–7.8 billion years, suggesting that life on the planet is already about 70 per cent of the way through its run. Other planets - especially those that form near the outer boundary of a star’s habitable zone or orbit long-lived, low-mass stars - may have habitable-zone lifetimes of 42 billion years or longer.
The authors suggest that scientists searching for life on other planets should focus on those that have occupied their habitable zones for at least as long as Earth has - such as HD40307g, which is 12.9 parsecs (42 light-years) away from Earth.
Meanwhile, a magnitude 8.3 earthquake that struck deep beneath the Sea of Okhotsk on May 24, 2013, has left seismologists struggling to explain how it happened.
According to the study published Monday in Science, at a depth of about 609 kilometers (378 miles), the intense pressure on the fault should inhibit the kind of rupture that took place.
The research team looked to the stars for inspiration. Using recently discovered planets outside our solar system (exoplanets) as examples, they investigated the potential for these planets to host life.
Andrew Rushby led the research, from UEA’s School of Environmental Sciences.
Rushby said: “We used the ‘habitable zone’ concept to make these estimates - this is the distance from a planet’s star at which temperatures are conducive to having liquid water on the surface.”
“We used stellar evolution models to estimate the end of a planet’s habitable lifetime by determining when it will no longer be in the habitable zone. We estimate that Earth will cease to be habitable somewhere between 1.75 and 3.25 billion years from now. After this point, Earth will be in the ‘hot zone’ of the sun, with temperatures so high that the seas would evaporate. We would see a catastrophic and terminal extinction event for all life.
“Of course conditions for humans and other complex life will become impossible much sooner - and this is being accelerated by anthropogenic climate change. Humans would be in trouble with even a small increase in temperature, and near the end only microbes in niche environments would be able to endure the heat.
“Looking back a similar amount of time, we know that there was cellular life on earth. We had insects 400 million years ago, dinosaurs 300 million years ago and flowering plants 130 million years ago. Anatomically modern humans have only been around for the last 200,000 years - so you can see it takes a really long time for intelligent life to develop.
“The amount of habitable time on a planet is very important because it tells us about the potential for the evolution of complex life - which is likely to require a longer period of habitable conditions.
“Looking at habitability metrics is useful because it allows us to investigate the potential for other planets to host life, and understand the stage that life may be at elsewhere in the galaxy.
“Of course, much of evolution is down to luck, so this isn’t concrete, but we know that complex, intelligent species like humans could not emerge after only a few million years because it took us 75 per cent of the entire habitable lifetime of this planet to evolve. We think it will probably be a similar story elsewhere.”
Professor of Earth and planetary sciences at the University of California, Santa Cruz, United States, Thorne Lay, said: “It’s a mystery how these earthquakes happen. How can rock slide against rock so fast while squeezed by the pressure from 610 kilometers of overlying rock.”
Lay is coauthor of a paper, published in the September 20 issue of Science, analysing the seismic waves from the Sea of Okhotsk earthquake. First author Lingling Ye, a graduate student working with Lay at UC Santa Cruz, led the seismic analysis, which revealed that this was the largest deep earthquake ever recorded, with a seismic moment 30 per cent larger than that of the next largest, a 1994 earthquake 637 kilometers beneath Bolivia.
Astronomers have identified almost 1,000 planets outside our solar system. The research team looked at some of these as examples, and studied the evolving nature of planetary habitability over astronomical and geological time.
“Interestingly, not many other predictions based on the habitable zone alone were available, which is why we decided to work on a method for this. Other scientists have used complex models to make estimates for the Earth alone, but these are not suitable for applying to other planets.
“We compared Earth to eight planets which are currently in their habitable phase, including Mars. We found that planets orbiting smaller mass stars tend to have longer habitable zone lifetimes.
“One of the planets that we applied our model to is Kepler 22b, which has a habitable lifetime of 4.3 to 6.1 billion years. Even more surprising is Gliese 581d, which has a massive habitable lifetime of between 42.4 to 54.7 billion years. This planet may be warm and pleasant for 10 times the entire time that our solar system has existed!
“To date, no true Earth analogue planet has been detected. But it is possible that there will be a habitable, Earth-like planet within 10 light-years, which is very close in astronomical terms. However reaching it would take hundreds of thousands of years with our current technology.
“If we ever needed to move to another planet, Mars is probably our best bet. It’s very close and will remain in the habitable zone until the end of the Sun’s lifetime — six billion years from now.”
Meanwhile, deep earthquakes occur in the transition zone between the upper mantle and lower mantle, from 400 to 700 kilometers below the surface. They result from stress in a deep sub ducted slab where one plate of Earth’s crust dives beneath another plate. Such deep earthquakes usually don’t cause enough shaking on the surface to be hazardous, but scientifically they are of great interest.
The energy released by the Sea of Okhotsk earthquake produced vibrations recorded by several thousand seismic stations around the world. Ye, Lay, and their coauthors determined that it released three times as much energy as the 1994 Bolivia earthquake, comparable to a 35 megaton TNT explosion. The rupture area and rupture velocity were also much larger. The rupture extended about 180 kilometers, by far the longest rupture for any deep earthquake recorded, Lay said. It involved shear faulting with a fast rupture velocity of about 4 kilometers per second (about 9,000 miles per hour), more like a conventional earthquake near the surface than other deep earthquakes. The fault slipped as much as 10 meters, with average slip of about two metres.
“It looks very similar to a shallow event, whereas the Bolivia earthquake ruptured very slowly and appears to have involved a different type of faulting, with deformation rather than rapid breaking and slippage of the rock,” Lay said.
The researchers attributed the dramatic differences between these two deep earthquakes to differences in the age and temperature of the sub-ducted slab. The sub ducted Pacific plate beneath the Sea of Okhotsk (located between the Kamchatka Peninsula and the Russian mainland) is a lot colder than the sub ducted slab where the 1994 Bolivia earthquake occurred.
“In the Bolivia event, the warmer slab resulted in a more ductile process with more deformation of the rock,” Lay said.
The Sea of Okhotsk earthquake may have involved re-rupture of a fault in the plate produced when the oceanic plate bent down into the Kuril-Kamchatka subduction zone as it began to sink. But the precise mechanism for initiating shear fracture under huge confining pressure remains unclear. The presence of fluid can lubricate the fault, but all of the fluids should have been squeezed out of the slab before it reached that depth.
“If the fault slips just a little, the friction could melt the rock and that could provide the fluid, so you would get a runaway thermal effect. But you still have to get it to start sliding,” Lay said. “Some transformation of mineral forms might give the initial kick, but we can’t directly detect that. We can only say that it looks a lot like a shallow event.”
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